Force sensing structures which use electrical sensors to produce a signal indicative of a load applied to the structure are commonly referred to as load cells. Historically, load cells have had accuracy limitations associated with using a metal member to carry the primary load and with the attachment of a strain sensor to the metal member to measure the load. Stress induced strain instabilities in the metal have produced signal errors. The attachment of the strain sensor with adhesives and simple beamlike spring structures also produce signal errors and make it difficult to obtain a stable and repeatable signal proportional to the load applied to the load cell.
The primary load carrying element (PLC) is generally made of a metal which becomes less perfectly elastic and more dimensional unstable as the stress in the metal is increased and which has a temperature expansion characteristic different from that of the load cell strain sensor. Each of these effects generally causes signal error. Additionally, the interface between the strain sensor and the metal is not perfectly stress free or stable. This also produces signal errors.
There are other designs which use vibrating elements (generally crystals) to directly carry the majority of the load which is to be measured. The crystals act directly as force sensors (and not as strain sensors sensing deflection of a PLC). This minimizes the above-stated errors which would be present if a metal load carrying member were used. Vibrating crystal force sensor designs are, however, directly and seriously limited by the load limits of the crystals (which are brittle and best operated at a small fraction of the material stress test limit). For these reasons, such devices have a very limited ability to withstand loads beyond that load which will produce a fall signal (i.e., over-range loads) unless the crystal is configured to give only a small signal. That can be done only at the expense of a serious loss of accuracy. Such devices also have a comparatively highly stressed joint between the metal and the crystals, and this can produce errors or give reliability problems. Still other designs use either an analog strain gage or a vibrating element strain gage (generally a crystal) glued to the metal to transfer the strain from the metal to the strain gage. The glue joint can produce both temperature and time dependent signal errors.
U.S. Pat. No. 5,313,023 discloses a load cell which uses two sets of cantilevered parallelogram spring structures fabricated in a monolithic structure for mounting a pair of vibrating crystal strain gages of a form referred to as a double ended tuning fork (see U.S. Pat. No. 4,215,570).
The crystals are mounted on surfaces which are at a right angle to the direction of motion of the springs and parallel to the strain sensor sensing axis. This requires parallelogram
like structures to keep the mounting surfaces parallel to the direction of motion. This arrangement is typical and is still very dependent on a bonded interface joint to carry the signal strain to the strain sensors. The crystal attachment in this type of device experiences a shear load, and any creep in that joint will produce some signal error.
The load cell disclosed in U.S. Pat. No. 5,313,023 also uses a cantilever beam structure which results in large moment stresses being present in the mounting of the load cell. This can produce signal error and potential mounting problems for the user.
Also previously proposed have been load cells of the ring type. The approach in these prior art cells is to place the sensing element directly across the ring both in the case of single ring and double ring designs. This approach limits the strain signal sensor to measurement of strains in the rings only and limits the options of the designer in optimizing all the essential characteristics of the load cell such as stiffness, stress, and signal characteristics.
There are other prior art transducer designs which use simple beam structures to preload a strain sensing crystal, adjust the transducer capacity, and avoid the use of glue joints to directly carry the signal component of strain to the strain sensors. Such beam structures are generally not integral to the primary load carrying structure but instead preload crystals between separate spring members which strain in response to the applied load. The use of separate pieces for preloading can cause instabilities and signal errors.
There is consequently an existent, continuing need for load sensors which do not have the above-discussed disadvantages of those heretofore proposed designs which employ metal load carrying elements, vibrating crystal load sensing components, and other prior art approaches to load measurement.